Titanium oxide photocatalysts can exhibit strong oxidation action through the absorption of ultraviolet rays and have been recently used in various applications as follows:
1. air purification by removing environmental pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx), which are emitted typically as automobile exhaust gases;
2. deodorization by removing malodor-causative substances such as ammonia, acetaldehyde, hydrogen sulfide, and methyl mercaptan;
3. water purification by decomposing and removing organochlorine compounds such as tetrachloroethylene and trihalomethanes;
4. antimicrobial action by killing microorganisms and further decomposing the dead microorganisms; and
5. antifouling action by decomposing oils to prevent soiling caused by the attachment of sand and dirt to the oils.
Such titanium oxide photocatalysts may be used as suspensions in a solution or as being supported on a substrate. The photocatalytic activity of a photocatalyst is generally in proportional to the surface area thereof, and the suspended titanium oxide photocatalyst has higher activity than that of the supported titanium oxide photocatalyst. However, the latter (supported titanium oxide photocatalyst) is employed more often than the former (suspended titanium oxide photocatalyst) from the viewpoint of practical utility. When the latter is employed, the titanium oxide photocatalyst is supported on a substrate generally by bonding the same with the substrate by the action of a binder component.
When the titanium oxide photocatalyst is bonded with the substrate using a binder component, however, the resulting supported catalyst disadvantageously fails to ensure satisfactory durability. This is because the binder component is decomposed with time through oxidation by the titanium oxide particles, loses its adhesion, and peels off from the substrate surface.
Citation 1 describes that a photocatalytic coating film capable of exhibiting satisfactory adhesion over the long term can be formed by employing granular anatase titanium dioxide as a photocatalyst; and blending the photocatalyst with titanium peroxide serving as a binder component, which titanium peroxide is resistant to decomposition by the titanium dioxide. However, the titanium oxide particles, if embedded in the binder component, exhibit insufficient photocatalytic activity. To prevent this, the coating film preferably has a very small thickness so as to expose the titanium oxide particles from the coating film surface. Still the coating film, unless it has a certain thickness, has insufficient adhesion and fails to be bonded with the substrate surface. Specifically, the coating film disadvantageously hardly has adhesion and photocatalytic activity both at satisfactory levels.